To cure various diseases caused when blood vessels or lumens are stenosed or closed, the stent is a tubular medical appliance to be implanted in lumens of the living body to expand the stenosed or closed portion and thereby secure the passage.
The stent is inserted into the body from the outside. Therefore the stent is so constructed that its diameter is relatively small when it is inserted into the body and is enlarged at a desired stenosed or closed portion by expanding the stent. The stent then maintains the expanded state to keep the lumen open.
The stent is cylindrical and made of a metal wire or a processed metal pipe. After the stent is mounted on a catheter or the like and the diameter of the stent is decreased, the stent is inserted into the body. Thereafter, the stent is expanded at the desired portion or location by using an expanding method and is fixed to the inner wall of the lumen of the desired portion, with the stent in close contact with the lumen inner wall to maintain the configuration of the lumen. Stents are classified into self-expandable stents and balloon expandable stents, depending upon the function of the stent and the implantation method. The balloon expandable stent, which has no self-expanding function or capability, is located at a desired portion. Then, a balloon positioned inside the stent is inflated to outwardly expand (plastically deform) the stent by an expansive force of the balloon so that the stent is fixed to the inner surface of the desired lumen with the stent in close contact with the lumen inner surface. Thus, to implant this type of stent to the desired portion of the living body, It is necessary to perform an operation of expanding the stent. The balloon expandable stent is primarily used as the stent to cure blood vessels and particularly the coronary arteries. The stent must have an axially flexible construction to cope with a variety of situations.
Balloon expandable stents are classified into closed cell types and opened cell types, depending on the configuration of the strut of the stent. The balloon expandable stent of the opened cell type is advantageous in that it is flexible. Thus the balloon expandable stent of the opened cell type can be favorably implanted in a desired portion because it is capable of flexibly following a travel direction (direction of extent) of a blood vessel and its configuration. Thereby it is possible to prevent the blood vessel from being stimulated. But the balloon expandable stent of the opened cell type has the disadvantage that the stratum of the stent flares outwardly. On the other hand, the balloon expandable stent of the closed cell type is advantageous in that the stratum of the stent does not flare outwardly. However, this type of stent suffers from the disadvantage that it is incapable of flexibly following the travel direction or direction of extent of the blood vessel and its configuration. Thus, balloon expandable stents of the opened cell type and the closed cell type have both advantages and disadvantages. Thus it is necessary to use the balloon expandable stent of the opened cell type or the closed cell type depending upon the direction of extent and the configuration of the blood vessel.
An example of the balloon expandable stent of the opened cell type is disclosed in Japanese Patent Application Laid-Open No. 2002-136601.
An example of a closed cell-type balloon expandable stent is disclosed in International Application Publication No. WO 96/03092 and Japanese Patent Application Laid-Open No. 10-503676 which corresponds to in International Application Publication No. WO 96/03092.
But in this closed cell-type stent, connection portions are disposed all over the stent. Thus this stent lacks flexibility and has a construction not well suited for following the inside of a blood vessel and being disposed at a curved portion.
In the balloon expandable stent of the opened cell type disclosed in Japanese Patent Application Laid-Open No. 2002-136601, the opened cell portion has a certain expanded-state retention force. However, it is desirable that the stent have a higher expanded-state retention force. It is also desirable that the stent have a low degree of a length change in its axial direction when it expands and has a higher follow-up performance for organs.
It is desirable that a living body-implantable stent possess a relatively low degree of axial length change during expansion, a relatively high follow-up performance for organs and a relatively high expanded-state retention force.